Hot dip coating apparatus

ABSTRACT

Hot dip coating apparatus for coating a steel strip in an Al—Zn alloy bath comprises components that are made from stainless steel containing an appreciable amount of nitrogen distributed substantially uniformly through its microstructure. The nitrogen is incorporated into the stainless steel as an alloy additive and improves the steel&#39;s corrosion resistance and minimizes pitting and thinning of the component when immersed in the metal bath for extended periods.

TECHNICAL FIELD

This invention relates to the continuous, hot dip coating of steel stripwith a coating alloy that contains aluminium. More particularly, theinvention relates to the in-bath components of apparatus used to effectsuch a coating process.

BACKGROUND ART

Traditionally steel strip was coated with zinc and was then referred toas galvanised steel. Zinc coatings have long been supplanted by coatingof an aluminium-zinc alloy. Such alloy coatings retain the sacrificialprotection afforded by zinc enhanced by the corrosion resistance ofaluminium. A typical coating alloy may nominally comprise 45% zinc and55% aluminium.

To effect the hot dip coating process the steel strip is drawn through apool of the molten coating alloy within an open topped bath. To controlthe passage of the strip into and then out of the pool of alloy,referred to hereinafter as the bath metal in accordance withconventional terminology, the strip is caused to pass under a sink rollsubmerged in said bath metal.

Conventionally the sink roll and its submerged supporting structure havebeen made of a corrosion resistant alloy steel, for example acommercially available steel designated grade 316L stainless steel. Evenso the working life of the submerged components is relatively short dueto the corrosive effect of the bath metal and the build-up ofintermetallic deposits resulting from chemical reaction between thecomponents and the bath metal.

Prior art FIGS. 1 and 2 of the accompanying drawings illustrate theresults arising from the use of 316L stainless steel.

FIG. 1 is a schematic diagram of the microstructure 50 of portion of asink roll of 316L stainless steel 51. It shows a deposit of a mixture ofbath metal 52 and intermetallic compound 53 on the surface of a normalalloy layer 54, which includes iron, chromium, nickel and aluminium, andwhich forms when the sink roll is immersed in the bath metal.

FIG. 1 also show the presence of σ-phase grain boundary precipitates 55.316L stainless steel 51, and most other stainless steels, aresusceptible to the formation of σ-phase precipitates over extendedimmersion times, which make the steel hard and brittle. Furthermore theσ-phase precipitates are rich in chromium and molybdenum so that theirgrowth causes depletion of those elements in the grains surrounding theσ-phase precipitates. The presence of such micro-cracks together withthe depletion of the overall chromium and molybdenum in the grains leadsto high dissolution rates of the steel when exposed to the molten bathmetal 52. Such dissolution manifests itself as pitting and other erosionof the submerged components.

Because the deleterious effect of depositing of the intermetalliccompounds 53 on the quality of the finished product it is necessary todress the roll from time to time to remove the deposit. This dressing isan expensive process, and it requires the coating operation to beinterrupted for the removal and replacement of the sink roll.

Prior art FIG. 2 illustrates a severely pitted sink roll support armfabricated from 316L stainless steel. Of course a sink roll, because itcontacts the strip being coated and the coating quality depends on thesmoothness of the roll, would have to be withdrawn from service longbefore it reached the state of the arm appearing in FIG. 2.

To overcome the deficiencies outlined above it has been proposed tosubject the sink roll to a nitriding process. Nitriding is aconventional process affecting a thin surface layer of the componentbeing nitrided and comprises holding the component for long periods in afurnace having an ammonia atmosphere.

When a sink roll that has been subjected to a nitriding process isimmersed in the bath metal, the nitrides react with the aluminium in thebath metal, so that in addition to forming the alloy layer, a layer ofaluminium nitride forms on its outer surface. This aluminium nitridelayer is stable and acts as a protective, adherent surface layer on thecomponent.

Prior art FIG. 3 is a view similar to FIG. 1 in respect of a nitrided316L stainless steel sink roll. The figure shows all of the features ofFIG. 1, but also shows a nitrided layer 56 with an aluminium nitridesurface layer between the mixture of bath metal 52 and intermetalliccompound 53 and the normal alloy layer 54 that forms when the sink rollis immersed in the bath metal. It will be noted that FIG. 3 also showsthe presence of σ-phase precipitates 55 in the microstructure.

Nitriding is beneficial in that the stable aluminium nitride layerrenders the intermetallic compounds less adherent to the roll. Thisfacilitates their removal by scraping and prolongs the periods betweendressings of the sink roll. The aluminium nitride layer also acts as aprotecting layer and limits pitting or erosion of the component. Thedisadvantage of the nitriding process is the expense, the expert abilityrequired to perform it, and the long wait required for obtaining thefinished component.

SUMMARY OF THE INVENTION

According to a first aspect, the present invention relates to a hot dipcoating apparatus for coating a steel strip wherein the strip isimmersed in a bath of coating alloy containing aluminium, the apparatusincluding at least one component having a surface that comes intocontact with the bath when in use, wherein the component is made fromstainless steel containing an appreciable amount of nitrogen distributedsubstantially uniformly throughout its microstructure.

The stainless steel used in this aspect of this invention differs fromthe prior art in that the nitrogen is present as an alloy additive inthe stainless steel as distinct from being introduced as part of anitriding process. The inventors have found that such high nitrogenstainless steels exhibit improved corrosion resistance when immersed inthe bath metal.

When making components in accordance with the invention they may be useddirectly in the hot dip coating apparatus without the need for anypre-treatment such as a nitriding process. In addition, as the nitrogenis distributed throughout the stainless steel microstructure it is notrelying on the integrity of the outer surface layer of the component andis therefore considered to be more robust than prior art systems.

In one form, the stainless steel contains greater than 0.10 wt % ofnitrogen. The inventors have found that concentrations greater than 0.10wt % nitrogen exhibit the improved properties which are characteristicof the invention. Austenitic stainless steel which contains nitrogen inthe above quantities is commercially available, such as that designatedby steel merchants as 316LN.

In one form, the entire component can be made from the stainless steelcontaining the appreciable amount of nitrogen. In another form, thecomponent may be manufactured as a composite structure with thestainless steel containing the nitrogen being used as an outer layer ofthe component. In this example, the component may include a furtherinner layer. This further layer may be formed of any suitable materialsuch as conventional stainless steel such as 316L. This latter form ofthe invention may be used where the component uses the high nitrogenstainless steel as a protective coating. Such an arrangement may beemployed where the component is being relined, or to reduce cost byusing a less expensive material as an inner core of the component. Inyet a further aspect, the invention relates to a hot dip coatingapparatus for coating a steel strip wherein the strip is immersed in abath of coating alloy containing aluminium, the apparatus including atleast one component having a surface that comes into contact with thebath when in use, wherein the component includes at least one layer madefrom stainless steel containing an appreciable amount of nitrogendistributed uniformly though its microstructure.

In one form, the component further comprises a further layer wherein thestainless steel layer containing the nitrogen is disposed between theouter surface and the further layer.

In one particular embodiment, the component is a sink roll under whichthe metal strip is passed.

In yet a further aspect, the invention relates to a method of forming acomponent of a hot dip apparatus for immersing a sheet metal strip in abath of coating alloy containing aluminium, wherein the component isformed at least in part from a stainless steel containing an appreciableamount of nitrogen, the nitrogen being dissolved into the stainlesssteel whilst in a molten state so as to be substantially distributedthroughout its microstructure.

In yet a further aspect, the invention relates to a method of coating asteel strip wherein the strip is immersed in a bath of coating alloycontaining aluminium, the method comprising the step of passing thesteel strip over a component immersed in the bath, wherein the componentis made from stainless steel containing an appreciable amount ofnitrogen distributed substantially uniformly through its microstructure.

BRIEF DESCRIPTION OF THE DRAWINGS

It is convenient to hereinafter describe an embodiment of the presentinvention with reference to the accompanying drawings. It is to beappreciated that the particularity of the drawings and the relateddescription is to be understood as not superseding the generality of thebroad description of the invention.

In the drawings:

FIG. 1 is a schematic diagram of a microstructure of a sink roll formedfrom a 316L stainless steel;

FIG. 2 is a photograph of a severely pitted sink roll support armfabricated from 316L stainless steel;

FIG. 3 is a schematic diagram of the microstructure of a sink roll froma nitrided 316L stainless steel;

FIG. 4 is a schematic illustration of a hot dip coating apparatus;

FIG. 5 is a schematic diagram of a microstructure of a sink roll formedfrom a high nitrogen stainless steel;

FIG. 6 is a schematic diagram of a microstructure of a sink roll formedfrom a 316L stainless steel and high nitrogen stainless steel;

FIG. 7 is a photograph of the surface appearance of immersion samplesformed from 316LN stainless steel after 1, 3 and 4 months; and

FIG. 8 is a parabolic plot of alloy layer growth for the samplesimmersed for 2 weeks, 1, 3 and 4 months.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 4 is a schematic illustration of a hot dip coating apparatus 10.The coating apparatus includes a pot 11 which incorporates a pool ofmolten coating alloy (the bath metal) 12. The pot 11 is open topped andis arranged to receive a steel strip 100 which is drawn through the bathmetal 12. To control the passage of the strip 100 into and then out ofthe bath metal 12, the strip is caused to pass through a snout 13, thenunder a sink roll 14 submerged in the bath metal, and then throughstabilising rolls 15 before leaving the bath metal.

To improve the corrosion resistance of the hot dipped coating apparatus10, at least some of the in bath components and, in particular, the sinkroll 14, is formed from a high nitrogen stainless steel. Othercomponents, such as the stabilising rolls 15, the snout 13 or thesupport arms and bearings for the sink roll 14 or stabling rolls 15 mayalso be made from a high nitrogen stainless steel. The nitrogen isincorporated as an alloy additive into the stainless steel whilst in itsmolten state so that it is distributed substantially uniformlythroughout its microstructure.

FIG. 5 is a schematic diagram of the microstructure 20 of a portion of acomponent of the apparatus 10, typically the sink roll 14. The componentis manufactured from a high nitrogen stainless steel which extends to anouter surface 21 which in use is exposed to the bath metal 12.

FIG. 6 illustrates an alternative arrangement where the component ismanufactured from a composite structure. FIG. 6 illustrates a schematicdiagram of the microstructure 22 of a portion of the component where aninner layer 23 is formed from a conventional stainless steel such as316L and an outer layer 24 which incorporates the outer surface 25 isformed from the high nitrogen stainless steel.

The following example illustrates the improved corrosion resistanceusing the high nitrogen stainless steel.

EXAMPLE

Immersion tests of samples of a 316LN stainless steel alloy wereconducted in a 55% AL—ZN alloy bath. The tests were conducted over a 4month period and samples were removed from the bath after immersion for2 weeks, 1, 3 and 4 months.

316LN alloy is a nitrogen containing austenitic stainless steel and itscomposition is as follows: Steel Type C Mn Si Cr Ni P S Mo N 316LN 0.032.0 1.0 16.0- 10.0- 0.045 0.03 2.0- 0.10- 18.0 14.0 3.0 0.16

FIG. 5 is a photograph of the surface appearance of the 316LN immersionsamples 30, 31 and 32 after 1, 3 and 4 months after continuous immersionin the metal bath. Visual examination showed no evidence of erosion orlocalised pitting or thinning of the edges of the sample. In addition,the surface of the samples showed no evidence of spiked or horned growth(ie. cone-shaped alloy outbreaks on the surface of the immersed potgear). Spike growth is related to the presence of σ-phase in themicrostructure of the pot gear.

The immersion samples also reacted with the bath metal and formed analloy layer similar in composition to that found in 316L. FIG. 6 showsthe alloy growth as a function of the square root time of immersion. Thegraph indicates that the alloy growth rate is diffusion controlled.

Accordingly, the use of a high nitrogen stainless steel, where thenitrogen is introduced into the melt (as distinct from a nitridingprocess) exhibits enhanced performance as compared to conventional 316Lstainless steel. Whilst test conducted to date clearly illustrate theenhanced performance in using high nitrogen stainless steel ascomponents in a hot dip apparatus, the mechanism by which thoseimprovements are obtained is not certain. Nevertheless, whilst notbinding the invention to theory, the inventors consider that onecontributing factor for the enhanced performance is that the nitrogenwithin the microstructure of the high nitrogen stainless steel is ableto move sufficiently freely so as able to move to the surface where itcan react with the aluminium to form an outer layer of aluminiumnitride. A further mechanism that may contribute to the improvedperformance is through the nitrogen restricting the growth of theσ-phase precipitates. The underlying cause of for σ-phase precipitationin austenitic stainless steels such as 316L is associated with thepresence of a small amount of δ-ferrite phase in the microstructure. Thepresence of δ-ferrite in 316L promotes the precipitation of σ-phase inthe microstructure of 316L after extended time of exposure at operatingbath temperature. Nitrogen is an austenite stabiliser and the additionof nitrogen as an alloying addition significantly reduces the level ofδ-ferrite in the stainless steel. Furthermore, increasing the nitrogencontent of the alloy increases the resistance of the alloy to localisedcorrosion like pitting or intergranular corrosion.

Whilst the use of a 316LN stainless steel has been used, it isconsidered that other compositions of commercially available steel mayalso provide the enhanced performance. The following table sets forthcompositions of other commercially available steels containingappreciable amounts of nitrogen distributed substantially uniformthroughout their microstructure at levels equal to or more than that ofthe 316LN, and which are therefore also considered to be of use in theapparatus of the present invention. Steel Type C Mn Si Cr Ni P S Mo N201 0.15  5.5- 1.0  16.0-  3.5- 0.06 0.03 — 0.25 7.5 18.0 5.5 202 0.15 7.5- 1.0  17.0-  4.0- 0.06 0.06 — 0.25 10.1  19.0 6.0 205 0.12- 14.0-1.0  16.5-  1.0- 0.0 0.03 — 0.10- 0.25 15.5  18.0  1.75 0.16 304LN 0.032.0 1.0  18.0-  8.0- 0.045 0.03 — 0.10- 20.0 12.0  0.16 304N 0.08 2.01.0  18.0-  8.0- 0.045 0.03 — 0.10- 20.1 10.5  0.16 316LN 0.03 2.0 1.0 16.0- 10.0- 0.045 0.03 2.0- 0.10- 18.0 14.0  3.0 0.16 316N 0.08 2.0 1.0 16.0- 10.0- 0.045 0.03 2.0- 0.10- 18.0 14.0  3.0 0.16

Accordingly, the present invention provides components for a hot dipcoating apparatus which have improved corrosion resistance through theuse of high nitrogen stainless steel. Whilst an advantage of the presentinvention is that it can obviate the need for separate pre-treatment,such as a separate nitriding process, it is to be appreciated that ifnecessary, the present invention in one form may also be used inconjunction with such processes. For example, such an arrangement may beused to provide a nitrided layer adjacent the outer surface of thecomponent so as to ensure that an outer layer of aluminium nitride formson immersion on the component into the molten bath. In that application,the aluminium nitride layer would allow for easier removal of the buildup of intermetallic compounds on the surface. The nitrogen within themicrostructure of the stainless steel could inhibit the growth of theσ-phase precipitates and also provide a feed of nitrogen to the outerlayer so that the aluminium nitride could be regenerated if it isbroken.

A further advantage of the present invention, is that considerably moredressing of the roll may be effected than in the case with a nitridedroll made of conventional steel devoid of nitrogen in its untreatedcomposition. This is because relatively few dressings of the prior artroll result in the complete removal of its nitrided layer, requiringrestoration of that layer via further nitriding operations.

In the claims which follow and in the preceding description of theinvention, except where the context requires otherwise due to expresslanguage or necessary implication, the word “comprise” or variationssuch as “comprises” or “comprising” is used in an inclusive sense, i.e.to specify the presence of the stated features but not to preclude thepresence or addition of further features in various embodiments of theinvention.

Variations and/or modifications may be made to the parts previouslydescribed without departing from the spirit or ambient of the presentinvention.

1. A hot dip coating apparatus for coating a steel strip wherein thestrip is immersed in a bath of coating alloy containing aluminum, theapparatus including at least one component having a surface that comesinto contact with the bath when in use, wherein the component is madefrom stainless steel containing an appreciable amount of nitrogendistributed substantially uniformly throughout its microstructure. 2.The hot dip coating apparatus according to claim 1, wherein thestainless steel contains greater than 0.10 wt % of nitrogen.
 3. The hotdip coating apparatus according to claim 1, wherein the component is asink roll under which the metal strip is passed.
 4. The hot dip coatingapparatus for coating a steel strip wherein the strip is immersed in abath of coating alloy containing aluminum, the apparatus including atleast one component having a surface that comes into contact with thebath when in use, wherein the component includes at least one layer madefrom stainless steel containing an appreciable amount of nitrogendistributed uniformly through it microstructure.
 5. The hot dip coatingapparatus according to claim 4, wherein the stainless steel containsgreater than 0.10 wt % of nitrogen.
 6. The hot dip coating apparatusaccording to claim 4, wherein the component includes a further layer,and wherein the stainless steel layer containing the nitrogen isdisposed between the surface and the further layer.
 7. The hot dipcoating apparatus according to claim 6, wherein the further layer isformed from stainless steel.
 8. A component for a hot dip coatingapparatus for coating a steel strip wherein the strip is immersed in abath of coating alloy containing aluminum, the component having asurface that comes into contact with the bath when in use, and is madeat least in part from stainless steel containing an appreciable amountof nitrogen distributed substantially uniformly throughout itsmicrostructure.
 9. A method of forming a component of a hot dipapparatus for immersing a sheet metal strip in a bath of coating alloycontaining aluminum, wherein the component is formed at least in partfrom a stainless steel containing an appreciable amount of nitrogen, thenitrogen being dissolved into the stainless steel in a molten state soas to be substantially distributed throughout its microstructure.
 10. Amethod of coating a steel strip wherein the strip is immersed in a bathof coating alloy containing aluminum, the method comprising the step ofpassing the steel strip over a component immersed in the bath, whereinthe component is made from stainless steel containing an appreciableamount of nitrogen distributed substantially uniformly through itsmicrostructure.
 11. The hot dip coating apparatus according to claim 2,wherein the component is a sink roll under which the metal strip ispassed.
 12. The hot dip coating apparatus according to claim 5, whereinthe component includes a further layer, and wherein the stainless steellayer containing the nitrogen is disposed between the surface and thefurther layer.
 13. The component for a hot dip coating apparatusaccording to claim 8, wherein the stainless steel contains greater than0.10 wt % of nitrogen.
 14. A component for a hot dip coating apparatusfor coating a steel strip wherein the strip is immersed in a bath ofcoating alloy containing aluminum, the component having a surface thatcomes into contact with the bath when in use, wherein the componentincludes at least one layer made from stainless steel containing anappreciable amount of nitrogen distributed uniformly through itsmicrostructure.
 15. The component for a hot dip coating apparatusaccording to claim 14, wherein the stainless steel contains greater than0.10 wt % of nitrogen.
 16. The component for a hot dip coating apparatusaccording to claim 14, wherein the component includes a further layer,and wherein the stainless steel layer containing the nitrogen isdisposed between the surface and the further layer.
 17. The componentfor a hot dip coating apparatus according to claim 16, wherein thefurther layer is formed from stainless steel.